Among the examples of previously reported water-soluble compound membranes having different structures on the front and back are laminates of langmuir membranes (LB membranes) with a hydrophobic functional group and a hydrophilic functional group arranged at the gas-solid interface and the solid-liquid interface, respectively, and alternate laminate membranes of a cationic polymer and an anionic polymer.
However, fabrication of a self-supporting membrane for functional membrane applications requires laminating LB membranes or alternate laminate membranes of a molecular length thickness in several hundred layers. This is time consuming and costly.
There is a method in which a thin membrane prepared by using an existing technique such as spin casting is crosslinked to become insoluble to water. However, this requires restricting the starting material compounds, or mixing different compounds. It is also difficult to form a self-supporting membrane having different front and back structures because the whole membrane is crosslinked to become insoluble to water.
A plasma process by plasma or electron beam irradiation has been known for many years since it was first reported some 140 years ago (Non Patent Literatures 1 and 2), and plasma irradiation is commonly used in applications such as deposition from a gas phase using intermolecular radical polymerization, a surface hydrophilic treatment, and crosslinking of polymers. For example, Patent Literature 1 describes performing plasma gas phase deposition by heating polysaccharides or methyl silicon resins to evaporate, and introducing the gas onto a substrate with an inert gas or other materials.